The present invention relates to improvements in the wet processing of leather. More particularly, the invention is directed to novel tanning agents and methods for making and using the same in the tanning and retanning of leather.
The production of side leather has many variations depending upon the desired end product, characteristics of the hides, regional variations, individual preferences of tanners, and other factors, but generally passes through the following basic sequence;
After salting or brine curing of the raw hides by the meat packing houses, the hides arrive at the hide house of the tannery where they are opened, trimmed, and sorted. The hides then pass to the beam house where they are soaked for up to several days in water to swell the hide and fibers.
The major objective of the beam house is to remove hair, flesh and extraneous proteins and to rehydrate and swell the hide fibers so that they may be properly tanned. The unhairing process may be done by dissolving or burning the hair or more preferably loosening and saving the hair. The latter method involves liming with slaked lime and sodium hydrosulfide.
The fleshed and unhaired hides next pass to the tanyard where the wet processing of the leather is carried out in rotating drums called tanning wheels. The initial wet processing steps usually involve deliming by reacting with an acid salt, bating with enzyme salts, and optionally pickling with an acid. After these initial wet processing steps, the hides then receive their initial tanning with a chrome tanning agent. It will of course be understood that the various wet processing steps indicated above are interspersed with washings and floatings of the leather stock in water, frequently with light acid or alkaline treatments to adjust the pH.
After sorting, splitting and shaving the leather sides to the proper weight and thickness, the leather stock is subjected to a further wet processing step called retanning. Retanning involves the addition of additional chrome tanning agents, vegetable tanning agents and/or resin tanning agents or syntans. In addition, dyes, fat liquors, and any other materials desired in the final leather are added during this final wet processing step of retanning.
After the retanning, the leather sides are dried, conditioned and finished in various manners depending upon the desired end product. A more detailed description of each of the above steps in the production of side leather may be found for example in The Chemistry And Technology Of Leather, edited by O'Flaherty, Roddy and Lollar, Volume 3, pages 235-265, Reinhold Publishing Corporation 1962.
The present invention relates to the wet processing steps in the production of side leather, and more particularly to the initial tanning and retanning of chrome tanned leather. These steps take place in rotating drums or tanning wheels having capacities of 900 to 5,000 or more pounds of leather with corresponding amounts of water.
Chromium is used in the tanning of leather since it will form trivalent compounds such as chromium chloride, chromium sulphate, and chromium oxide. The most widely used chrome tannages are those with a chromium sulphate base. The chromium contents of the salt are usually stated in terms of chromium oxide (Cr.sub.2 O.sub.3), and commercially available chromium salts have about 25 percent chromium oxide. Commercial chromic or chromium sulphate comes in various hydrated forms which include additional water molecules in complex chemical linkages, represented by the general formula Cr.sub.2 (SO.sub.4).sub.3 .sup.. XH.sub.2 O.
The existence of hydroxyl groups from water in the chromium sulphate, referred as the degree of basicity, can range from monobasic to dibasic and tribasic. The tribasic chromium sulphate salt (having three hydroxyl groups in the formula Cr.sub.2 (OH).sub.3) is the most effective in tanning and is referred to as having a 33 percent basicity. The tribasic chromium sulphate has a small size molecular structure which is of great importance in chrome tanning since the particle size controls the degree and speed of penetration of the tanning agent. Moreover, the tribasic salt is cationic and has a tremendous affinity for the fiber protein of the leather.
In order to control the reaction of the chrome tanning agent, a masking agent should be present. Commercial chrome tanning compounds are often manufactured by reducing bichromate of soda with an organic compound such as glucose. A fairly large percentage of this glucose remains in the commercial chromium sulphate and acts as a masking agent. Other masking agents may be added during the tanning including salts, such as sodium salts, of formate, acetate or oxalate. These masking agents simply slow up the cationic action of the chromium salt and maintain a minimum particle size, which results in controlled penetration.
The execution of the initial chrome tannage is performed directly after the hides have been soaked, limed, bated, and in some instances pickled. The prepared hides are added to a rotating drum charged with 80 to 100 percent water and 2 to 3 percent common salt. After several minutes of drumming the hides (agitating the hides in the water by rotating the drums), the diluted chrome tanning agent is usually added in an amount calculated at about 2.5 to 3 percent chromium oxide (33 percent basic salt). The drumming then continues for 5 or 6 hours.
The end point of the tanning (i.e., the point at which a sufficient amount of tanning agent has been picked up by the leather) is determined by immersing a sample of the treated hide in boiling water. A properly tanned leather will not curl or perforate after several minutes of boiling. Although a number of adjustments and additional steps are used by various tanneries, the above is a simple method of the initial tanning of leather.
At this point, the leather will have a pH of about 3.4 to 3.7, and is then removed from the drum for aging. At this point the leather is considered in its "blue" state. During the aging, the leather is horsed (i.e., hung on a horse to drain) to allow a proper setting of the chrome. After sorting, splitting the sides to the proper weight and thickness, and shaving to even thickness, the leather is ready for retanning.
After replacing the leather in a drum the leather must be neutralized by adjusting the pH to about 4.5 to 5, such as by the addition of bicarbonate of soda. The general procedure in retanning is then to incorporate additional amounts of chrome into the leather, as well as the addition of various synthetic tanning agents such as the resin types or vegetable or natural tanning agents. Dyes, fat liquors and lubricating oils are also added during the retanning.
Thee overall objective in retanning of leather is to produce a marketable end product in a number of forms to cover a wide market. Important factors to be achieved are the firmness or pliability of the leather, the general appearance and feel of the grain, the general appearance and feel of the flesh side of the suede, and the ability of the side of leather to be transformed into side leather for shoes, pocketbooks, belts, etc., by the use of finishes such as acrylic emulsions, lacquers and dye coatings.
To achieve the above objectives, tanneries have devised various methods of satisfying their needs. It is generally considered in the industry that leather can be softened by high chrome retanning or in the alternative by normal chrome retanning (6-8 percent Cr.sub.2 O.sub.3 is the standard amount of chrome desired in the leather although amounts from 5-20 percent are not uncommon depending on the tannery) and extensive use of oil fat liquors. The fat liquors generally used are sulphonated sperm oil or synthetic sperm oil, along with or blended with minerals and soap. These oils are either anionic or cationic and will therefore exhaust on leather quite readily. In effect, they act as a filler and lubricate the fibers and are fixed on the fibers when the leather is dried in the standard methods. Also, the use of fat liquors generally tends to increase the tear strength of the leather.
A serious problem in the leather industry in standard procedures for the initial chrome tanning is that as much as 25 to 30 percent of the chrome tannage or tanning agent does not exhaust from the tanning bath onto the hide fibers. As a result, this large amount of chrome tannage is lost on draining the drums after the initial tanning. This loss of tanning agent represents not only a great additional expense in materials, but also presents a serious effluent problem. Thus, chrome waste from tanneries constitutes a serious polluter of rivers and waterways and requires appreciable industry expense to control this pollution.
Accordingly, it would be exceedingly desirable to find a method of insuring complete or nearly complete exhaustion of the chrome into the leather.
In addition, it would be desirable to find a new method of obtaining a soft, strong leather in such a way that chrome tanning agents and/or fat liquors could be greatly reduced or eliminated during the retanning of the leather.
It would also be desirable to find an aqueous tanning agent or auxiliary for the chrome tanning of leather which would increase the dye value of dyes used in leathers, thereby permitting the use of reduced amounts of dye or improved depth of color with the same amounts of dye.